1. Field of the Invention
This invention relates generally to the field of polymer electrolytes. More specifically, the invention relates to the use of polymerizable salt surfactants as a nanoporous polymer electrolyte.
2. Background of the Invention
Many portable electronic devices as well as electric, hybrid and fuel cell vehicles require high performance rechargeable batteries. Presently, lithium batteries are the battery of choice due to its high energy density and power. The key to market success for electric vehicles is the energy storage device, which limits driving distance and acceleration. Rechargeable lithium batteries are the most promising technology for storing energy and delivering it on demand for electric vehicles because lithium batteries potentially have high energy densities (400 Wh/kg) and high power densities (800 W/kg), and therefore can meet, in principle, all of the performance requirements.
One aspect of the lithium battery assembly that needs to be improved in order to make rechargeable battery performance suitable for applications such as electric vehicles is the electrolyte. The electrolyte usually comprises a separator material and the electrolyte itself. The separator material allows lithium ion exchange, but prevents electrical conduction between the anode and cathode. The electrolyte is generally a lithium salt (such as LiCF3SO3 or LiPF6) dissolved in an organic solvent (for example ethylene carbonate and propylene carbonate), while the separator material is usually a polymer, although there are many variations, ranging from solvent in polymer “gels” to solvent-free polymer electrolytes. Solvent-based batteries often contain flammable liquid and are potentially unsafe. Additionally, solvents tend to participate in undesired reactions at the battery electrodes and can leak out of the casing.
Conversely, solvent-free polymer systems, such as polyethylene oxide (PEO) with lithium salts are safer, but have inherently low ionic conductivity, especially at low temperatures (i.e. lower than 10−8 S/cm at −40° C.). It is desirable to have an electrolyte/separator material for battery systems (in electric vehicles for example) that is polymeric, has a high Li+ capacity (concentration) and a usefully high Li+ conduction at temperatures ranging from −40 to 85° C.
Most polymer electrolytes developed to date have been based primarily on alkyl-ethers such as polyethylene oxide (PEO) modified with lithium salts. These electrolytes are not stable enough to be used with metallic lithium anodes. Resistive layers form at the interface due to mobility of anions, and lithium metal particles and dendrites form upon charging and discharging (which then migrate into the soft polymer electrolytes and form short circuits). Additionally, these electrolytes are dual ion conductors where ionic conduction is dominated by the anion and lithium transport accounts for only 30 to 50% of the total ionic conduction. In this type of electrolyte, ion conduction depends primarily on polymer segmental motion (i.e. thermal motion). However, polymer segmental motion is a function of temperature and the conductivity is significantly reduced at low temperature as the polymer motion decreases. Low temperature conductivity can be improved by adding non-aqueous liquid additives to the electrolyte, but this in not practical due to concerns about dimensional stability and leakage.
Consequently, there is a need for in the art for a polymer electrolyte that exhibits good room temperature conductivity and very little decrease in conductivity at low temperatures, without the addition of volatile solvents or plasticizers.